1. Technical Field
The present invention relates to devices and methods for use in orthopedic spine surgery. In particular, the present invention relates to a system that provides a low profile dynamic anterior vertebral body plate for the secure fixation and stabilization of the cervical spine without the need for additional small locking parts while permitting limited motion of the anterior plate.
2. Background Art
Disease, the effects of aging, or physical trauma resulting in damage to the spine has been treated in many instances by fixation or stabilization of the effected vertebra. The use of plates and screws for fixation and stabilization of the cervical vertebra has been widely accepted as a reliable practice and has proven to be highly successful clinically.
The various plates, which are attached to the anterior vertebral bodies of the spinal column by bone screws have some common features such as relatively planar body profiles that define multiple holes or slots through which the screws fit and are threaded into the bone. A common result of the use of conventional anterior vertebral plates is a rigid fixation of the vertebrae. This rigid fixation is not always a preferred outcome, such as when some motion of the plate relative to the bone would be desirable in cases where the height of the intervertebral space changes or when compression of the space between the vertebral bodies is desired. Innovations over time have been primarily directed to improving the dependable security of the bone screw to the plate and as such have resulted in virtually complete rigidity of the screw, bone plate, bone connection. Various means have been used to prevent the screws from becoming loose or detached from their necessary secured or locked attachment to the vertebral plate. Among the differences between the conventionally used plates and screws is the manner in which the screws are locked into place in the hole or slot of the plate after the screws have been secured to the bone.
These conventional devices can be generally grouped into three basic categories with regard to how the screws are captured or secured in the plates.
Early plate designs were standard bone plates having holes through which screws were passed and screwed into the bone. These plates had no special provision for attaching the screws to the plate and as such were susceptible to having the screws back out of the plate over time. There have been clinically reported instances of screws backing out of these type plates with resulting surgical complications. Due to the potential and actual unreliable performance of such plates, the need for secure fixation of the screw to the plate as well as to the bone is now considered a basic requirement for vertebral plates. Due to the lack of predictable security of the screw to the plate, plates which do not secure the screw relative to the plate have fallen out of favor and virtually disappeared from use.
Various efforts have been made to secure the screws relative to the plates. In one design the screw head contains a threaded hole configured to receive a set screw. After the screw has been driven into bone and the head is seated in the plate hole, the set screw is inserted into the receiving hole of the screw head. The set screw is tapered to cause the screw head to expand and frictionally engage the wall of the plate hole, thereby resisting forces which tend to cause the screw to back out. While such mechanisms have worked to some degree, the addition of a small additional part, the set screw, at the time of surgery presents the potential hazard of dropping the set screw into the surgical field or otherwise misapplying the set screw to the screw head, for example, cross threading. Further, these types of plates have not allowed for any limited motion of the plate relative to the screw so as to allow for the dynamics of a change in height of the vertebrae or for compression of the vertebrae.
An alternative approach has been to provide features in the plate, which are specifically designed to hold the screw in position once the screw is inserted through the plate and screwed into the bone. One direction taken in this effort has been to design plates that incorporate or attach individual retaining rings or snap features associated with each plate hole configured to hold the inserted screw in place relative to the plate. These plates are very common and widely used; however, an inherent problem associated with such plates is the use of the additional very small retaining elements that can become disengaged from the plate and migrate into the surrounding soft tissues. Further, difficulty experienced in accessing and disengaging the small locking elements and removing the screws from this type of plate has caused some concern for the continued use of such plates. A similar approach involves individual cams associated with each plate hole, which when rotated apply friction pressure to the screw head in an attempt to resist back out.
Another approach is to add a cover to the plate after the screws have been placed. Such a design undesirably adds steps to the surgical procedure, thickness or height to the overall construct, and is susceptible to misapplication. Yet another direction taken in this effort to provide plates with locking elements is to provide dedicated overlying features, which are attached to the top side of the vertebral plate for the purpose of covering at least a portion of the screw head and thereby holding the screw in a seated and locked position. Generally these plates are designed to provide a variety of screw covering features that are pre-attached to the plate, and which can be selectively slid or rotated into position once it has been inserted. In some devices, such covering plates cover multiple screw heads. These plates typically require an increase in the overall composite thickness of the plate in order to accommodate the additional locking feature attached to the top side of the plate. This is a particularly unacceptable condition due to the use of such plates in an area of the spine where a thin, smooth surfaced profile for the plate assembly is preferred. Another major problem with such plates is that the overlying locking element cannot always be properly positioned over the screw head if the screw shaft was, due to anatomical necessity, positioned through the plate and into the bone at an angle such that the screw head does not fully seat in the plate recess provided on the top side of the plate. Further, when one of the overlying locking elements of such a plate loosens or becomes disengaged it is then tree to float away from the top side of the plate and migrate into the soft tissue adjacent to the top side of the vertebral plate.
Yet another approach is to provide machine threads in the plate hole with corresponding threads on the screw head. Thus the screw has a first, bone engaging thread on its shaft and a second machine thread on the screw head. As the thread shaft is screwed into bone the screw head approaches the plate hole and the machine thread engages the thread in the hole. This approach to securely locking the bone screw into the plate shares the common fault of the other approaches discussed above; after the screw is locked to the plate, there is no allowance for movement of the plate relative to the underlying bone. As indicated above, in some cases, limited movement is desired to accommodate for a change in height of the intervertebral space or when compression of the space between the vertebral bodies is desired.
Common to all of the conventional anterior vertebral bone plates is the goal to provide a rigid secure fixation of the plate to the anterior surface of the vertebrae. In that effort the principal focus of innovation has been to prevent the bone screw from backing out of its connection with the plate and thus permitting undesired and damaging movement of the plate relative to the vertebrae. In some cases, such as to allow for the dynamics of change in the height between vertebral bodies or when compression of the space between the vertebral bodies is desired, it is preferred to use a securely attached dynamic anterior vertebral plate that permits limited directional motion of the screws relative to the plate. None of the above described conventional efforts to provide a secure connection of the screw to the anterior plate have attempted to address the need in certain cases to provide a dynamic or semi-constrained anterior bone plate that is intended to permit limited movement of the screw and plate connection, while at the same time providing a secure connection of the screw to the plate.
There is therefore, an unfulfilled need for an anterior cervical plate system that can maintain a relatively low profile, as found in the non-locking plates while providing the security of a locking plate system. Further there is a need for a vertebral plate that does not have additional separate locking elements with the predictable problems of locking elements becoming disengaged from the plate and migrating away from the top side of the plate into the surrounding soft tissue. Finally, there is a need to provide a dynamic or semi-constrained anterior vertebral plate that when fixed to the vertebrae can permit limited motion of the plate relative to the screw so as to allow for a change in height of the intervertebral space when compression of the space between the vertebral bodies is desired.